Production Of Particles In NEXUS Model

To study the formation of a quark gluon plasma, it is of crucial importance toundeIstand in detail the collision dynamics. Parton 6ascd GTi6ov-Regge theory,realized in the Monte Carlo code NEXUS, provides a consistent approach forhadron-hadron scattering and the initia1 stage of 11ucleus--nucleus collisioIls atultrarelativistic energies. The Monte Carlo simulation offers a systematic andefficient way to study this unknown realm by qualitative comparisons to theexperiments.The knowledge of these initial collision is crucial fOr any theoretical treat-ment of a possible parton-hadron phase transition, the detection of which beingthe ultimate aim of all the efforts of co1liding heavy ions at very high energies.The parton model allows to calculate inclusive cross sections as a convolution ofany elementary cross section with parton distribution functions, assuming thatfactorization works. The disadvantage is that tlle first interaction is coIlsideredquite differently from the subsequent ones in case of multiple scattering. AnotheraPproach, the Gri6ov-Regge theory allows multiple interactions to happen in par-allel. The energy needs to be shared among elementary interactions. That iswell taken into account when calculating particle production, but not when cal-culating cross section. So cross section and particle production are not calculatedconsistently. And it is difficult for Gri5ov-Regge theory to include hard process.The parton 6ased Gri6ov-Regge theory provide a solution: elementary scatteringShaPpen in parajlel, described phenomenologically as Pomeron exchanges. Apply-ing the general concept of field theory, one can treat both cross section calculationand particle production within the same formalism. The hard process is includedin a natural way, which offers the opportunity to calculate the parton spectra andparton density which aIe closely related to the fOrInatiOI1 of quark gluon plasma.The strict calculation is perfOrmed on the parton level, however, hadrons arefinally observable particles. So it is necessary to study in detail the hadronization,the transition from partons to hadrons. This is realized with purely phenomenth 一 3 logicalobjects,strln耶．Thetradltlonalstrlngmodelsfalllnfittingtheproton－ proton scatteringfrom NA49 coil劝oration:experimentally tmidr地iditythe IItiO Of yiyM E／E二 0．44士 0．08,hilt thOSS Stltw IOdolS giVV 5／5 ＝＝ 0．8… 1．2． The situation for os Is even more extreme:from extrapolatingA and三 results It is职poeted that W0二 0．5 N 0．8,but those string models give fi／fl二 1．6 NI．9, mu血 b屯驴rthan unity．It Is not aproblem ofa山ustingjust someparameters but a fundamental problem of those models:due to the very definition；the string ends are vdence quarks(but hot a sir朋ge quark),disfavour multi－strange baryon production nd let the models Incompatible with the data．The string formation mechanism In NEXUS3 gets a reasonable agreement with the data:日／三＝0．56 andn／0二 0．71．Here the inal state ofProtoll－Proton scattering Is a system of prOJectile and target remnant and a number of cut Pomerons represented by a pair of strings eaCh．The soft Pomerons,which dominate at SPS energy are v砒uum excitations and Produce Particles and antlpartlcles equally,with their string ends being sea quarks．The lenlllallts,collslsted of valellce quarks alld spectator quarks,favour leading baryon production．This picture explains the observed dl地rences In the rapidity spectra ofbaryons and antlbaryons as well s the measured baryon yields and the ratios ofmultl－stran罗 baryons to their ntip肛ticles．This picture lso o地rs a novel me山anlsm ofHO and s...